It has been known that addition of scandium (Sc) as an alloy element into aluminum (Al) or its alloy remarkably improves heat resistance. Therefore, in recent years, effective utilization of an aluminum based alloy (Al—Sc based alloy) has been expected in various industrial fields. Specifically, an aluminum based alloy obtained by adding Sc as an alloy element hardly exhibits a reduction in mechanical strength even after maintained at a temperature exceeding 200° C. for a long time period, even when the amount of an added rare earth element is 0.1 mass %. This is likely to be because, in the aluminum based alloy obtained by adding Sc as an alloy element, mechanical properties changed through plastic deformation processing hardly undergoes recovery or recrystallization through heating. For example, Patent Literature 1 introduces that an aluminum alloy material that is useful as a material for an aluminum based alloy wire and exhibits good conductivity, high mechanical strength, and excellent heat resistance can be obtained by incorporating 0.05 to 0.3 mass % of Sc and 0.1 to 0.4 mass % of Zr into a pure Al matrix.
However, such aluminum based alloy containing Sc has hitherto been utilized in extremely limited applications despite the promising utility in industrial fields. The reason for this is that metal Sc is liable to be oxidized and reduction for obtaining metal Sc from a scandium compound (Sc compound) such as a Sc halide or a Sc chalcogenide is associated with difficulties. Specifically, in order to obtain Sc as a metal, there is a need to reduce the Sc compound through heating using as a reducing agent an alkali metal such as Na, an alkaline earth metal such as Ca and Mg, or the like, which are more susceptible to oxidation than Sc, or to reduce the Sc compound through molten salt electrolysis.
For example, Patent Literature 2 discloses a technology for obtaining metal Sc powder, involving loading into a vacuum reaction vessel a Sc halide together with metal Ca, metal Zn, or the like, reducing the Sc halide with metal Ca to form a Sc—Zn alloy, separating the obtained Sc—Zn alloy phase from the halide phase containing an oxide of Ca, followed by pulverization, and further, performing oxidation treatment on the alloy powder obtained through pulverization to form a thin oxide coating on the powder surface, and thereafter, loading the resultant powder into a vacuum vessel containing an inert gas atmosphere, and performing heating under vacuum to vaporize Zn or the like serving as an alloy component. In addition, Patent Literature 3 discloses a technology for producing an aluminum based alloy containing a rare earth element by means of electrolytic reduction. The electrolytic reduction is conducted in a molten salt electrolytic bath mainly containing a fluoride of an alkali metal or an alkaline earth metal and a fluoride of a rare earth element such as Sc, Y, and lanthanoid by using as a cathode molten metal Al floating in an upper layer and protecting an anode with an insulating material so as to prevent the positive electrode from being brought into contact with the molten metal Al in the upper layer.
In the former reduction method through heating, the alkali metal or alkaline earth metal used as a reducing agent is expensive and its handling requires greatest care owing to remarkably high reactivity. Accordingly, there is a problem in that the reducing agent required for reduction cannot be mass-produced easily at low cost. In addition, in the latter reduction method by means of molten salt electrolysis, it is essential to utilize an electrolytic reduction vessel capable of withstanding high temperature exceeding the melting temperature of Al (660° C.), operation at high temperature is difficult for fear of contamination with impurities or the like from the vessel or the like, and the electrolytic reduction needs to be conducted at a temperature equal to or less than the melting temperature of Sc exceeding 1,500° C. (specifically, a temperature of 1,000° C. or less). In addition, there need some efforts to adjust reduction conditions so as to prevent Sc precipitated through reduction from becoming a solid metal to dendritically grow or to prevent Sc precipitated so as to form with another metal an alloy having a low melting temperature from becoming a solid. Further, equipment and power for electrolysis are required, the electrolytic bath externally heated needs to be operated in an inert gas atmosphere such as argon gas (Ar gas) in order that the molten metal Al (negative electrode) in the upper layer is not oxidized, and a time period for the electrolysis needs to be considered so as to prevent the specific gravity (substantially the same as “density”) of an generated aluminum alloy from exceeding the specific gravity of the molten salt. Accordingly, mass-production at low cost is not easily realized because of the great deal of labor and higher cost.
In this connection, with a view to producing an aluminum based alloy containing Sc, there have been proposed several technologies involving allowing a Sc compound as it is to react with metal Al to provide an aluminum based alloy containing Sc without conducting a step of reducing Sc from its compound form to a metal form.
For example, Patent Literature 4 discloses a technology for obtaining an aluminum based alloy containing a rare earth element through a reaction between a compound of a rare earth element and aluminum in the presence of a calcium chloride flux.
In addition, Patent Literature 5 discloses a technology for producing an Al—Sc alloy containing Sc at a high content, involving loading into a reaction vessel a Sc halide such as ScF3 and metal Al together with metal Ca serving as a reducing agent and LiF, CaCl2, or the like serving as a flux, performing heating at a temperature of from 800 to 1,000° C. in an inert gas atmosphere to reduce the Sc halide with metal Ca and concurrently allow the Sc halide to be alloyed with metal Al to form an Al—Sc alloy, and then, cooling the resultant at a cooling rate of from 10 to 70° C. per minute to the solidifying temperature of Al or to a temperature 100° C. lower than the solidifying temperature, thereby generating a precipitate with high density Sc and a precipitate with low density Sc in the Al—Sc alloy, and thereafter, separating the precipitate with high density Sc from the precipitate with low density Sc, and subjecting the precipitate with high density Sc to vacuum melting, thereby vaporizing residual metal Ca that is the reducing agent.
Further, Patent Literature 6 discloses a technology for obtaining a light metal alloy containing a rare earth element, involving mixing powder of an oxide or halide of a rare earth element such as Sc and powder of a light metal such as Al or Mg, pelletizing the obtained mixture through compression molding, and then, putting the pellet into a molten light metal bath after enhancing wettability of the surface of the pellet to the molten light metal, and reducing the oxide or halide of a rare earth element with the light metal.
Further, Patent Literature 7 discloses a technology for producing an aluminum based alloy containing a rare earth element that includes Y and lanthanoid that, involving subjecting an oxide of the rare earth element to a reaction with metal Al.